Abstract:

Dental articles are produced using relatively low sintering temperatures
to achieve high density dental articles exhibiting strengths equal to and
greater than about 700 MPa. Ceramic powders comprised of nanoparticulate
crystallites are used to manufacture dental articles. The ceramic powders
may include sintering agents, binders and other similar additives to aid
in the processing of the ceramic powder into a dental article. The
ceramic powders may be processed into dental articles using various
methods including, but not limited to, injection molding, gel-casting,
slip casting, or electroforming, hand, cad/camming and other various
rapid prototyping methods. The ceramic powder may be formed into a
suspension, pellet, feedstock material or a pre-sintered blank prior to
forming into the dental article.

Claims:

1. A dental article manufactured by the process comprising:mixing ceramic
powder comprising nanoparticles having a crystallite size of less than
about 20 nm with a sintering aid comprising metallic particles, wherein
the metallic particles have a crystallite size greater than about 100
nanometers;forming the powder mixture into a dental article;subjecting
the article to microwave radiation to densify the article.

2. The dental article of claim 1 further comprising mixing a binder with
the ceramic powder and sintering aid.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a divisional application of and claims priority
to U.S. application Ser. No. 12/490,539, filed Jun. 24, 2009, entitled
Methods of Manufacturing Dental Restorations Using Nanocrystalline
Materials, which is a divisional application of and claims priority to
U.S. application Ser. No. 10/857,482, filed May 28, 2004, entitled Dental
Restorations Using Nanocrystalline Materials and Methods of Manufacture,
which claims priority to U.S. Application No. 60/474,166 filed May 29,
2003, entitled Methods of Fabricating Dental Restorations Using
Nanocrystalline Materials, all of which are hereby incorporated by
reference.

[0003]Techniques of fabricating all-ceramic dental restorations by hand
and methods using commercial high-tech systems such as CAD/CAM systems
each have their limitations and target different segments of the dental
laboratory market. There are two main challenges restricting widespread
use of high-strength ceramic materials for cost-effective fabrication of
dental restorations and both challenges are related to the sintering step
of the operation. High-strength ceramic materials are crystalline
materials formed from powder and require high temperatures for sintering
that result in substantial shrinkage. Any technique enabling use of these
materials for dental restorations should offer ways to (1) compensate for
shrinkage and (2) provide a furnace capable of reaching the temperatures
necessary to sinter the material to nearly full density.

[0004]A technique reportedly providing the highest strength for manually
produced copings, the Vita® In-Ceram® method (developed by VITA
Zahnfabrik), has been advertised as yielding a material with flexural
strength of about 500 MPa or even higher. This technique has not become
popular mostly due to esthetic limitations and a tedious multi-step
fabrication procedure that includes a glass infiltration step. This glass
infiltration technique is one way to circumvent the above-mentioned
challenges. Vita® In-Ceram® copings are slip-cast on a gypsum die
and soft-sintered with negligible shrinkage. The final glass infiltration
step does not require a special furnace. The resulting product is a fully
dense restoration having undergone no shrinkage. Nonetheless, the
presence of a glass phase in the glass-infiltrated ceramics makes it
inferior to corresponding crystalline ceramics in mechanical strength and
chemical durability.

[0005]Currently available CAD/CAM systems are capable of compensating for
shrinkage by milling enlarged shapes. Moreover, high-temperature
isotropic sintering results in fully dense and accurate final shapes.
However, CAD/CAM systems and procedures are expensive and not affordable
by small labs. For example, two of the most recently developed commercial
state-of-the art CAD/CAM systems, the LAVA® system (available from 3M
ESPE) and the CERCON® system (available from Dentsply/Degussa),
require the purchase of a scanner, milling machine and high-temperature
sintering furnace and are currently priced in the range of approximately
$60,000-$180,000. Both of the aforementioned CAD/CAM systems employ
soft-sintered zirconia blocks. The blocks are milled to enlarged shapes
and subsequently sintered to full density. Both systems are advertised as
yielding materials having a flexural strength of about 900 MPa or higher.

[0006]Glass-ceramic materials obviate the need to compensate for shrinkage
and high temperature sintering. They can be hand-built on a refractory
die and sintered at fairly low temperatures to assure accuracy of the
final shape. One example of such a material is an OPC® Low Wear
(available from Pentron Laboratory Technologies, LLC) porcelain jacket
crown (PJC). Glass-ceramic materials can also be injection molded into a
refractory investment mold formed by the lost wax technique. Examples of
commercially available materials used in this process include OPC®
porcelain, and OPC® 3G® porcelain, IPS Empress® porcelain and
Empress 2® porcelain. The physical mechanism underlying the high
processability/formability of these glass-ceramics is the viscous flow of
its glass component. The glass-ceramic materials listed above (Optec®,
OPC® and OPC® 3G®, Empress® and Empress2® materials)
have from about 40% to about 60% of a glass phase which serves as a
matrix in which from about 40% (e.g., Optec) to about 60% of crystals
(e.g., Empress2) are embedded. These crystals are grown in-situ by
crystallization heat-treatment of the parent glass. Alternatively, in a
method described by Hoffman in U.S. Pat. Nos. 5,916,498, 5,849,068 and
6,126,732, in order to improve processability of the material, up to 50%
glass is added to the crystalline ceramic powder. As a result, the
reported flexure strength is limited to less than 600 MPa. By introducing
a glass phase into the microstructure, strength is compromised to gain
better processability.

[0007]Sintering of glass-ceramic powders is a relatively fast process
compared to sintering of crystalline ceramic powders due to the viscous
flow mechanism of the former, which is associated with higher
densification rates, but the presence of the residual glass phase limits
the strength of the final product. Another benefit of the viscous flow
mechanism is that the glass ceramic conforms to the shape of the die
during sintering without cracking. On the other hand, crystalline
ceramics can be much stronger than glass ceramics, but crystalline
ceramics sinter by a solid-state diffusion mechanism that is
intrinsically slow creating inhomogeneous shrinkage, generating
significant sintering stresses that may result in associated cracking.
Liquid phase sintering induced by the addition of sintering aids greatly
enhances sinterability of crystalline ceramics by promoting particle
rearrangement and solution-precipitation mechanisms but such mechanisms
do not achieve all the advantages of the viscous flow mechanism.

[0008]At the same time many experimental and theoretical studies reveal a
decrease of the melting temperature of nanometallic particles in
comparison with the melting temperature of conventional bulk metals. Its
magnitude depends mostly on particle size and crystal structure as well
as particle surface conditions and the host matrix environment such as
the presence of impurities, level of agglomeration, coating, deposition
substrate and the like. Usually, melting is associated with a pre-melting
process resulting in a change in shape of the nanoparticles followed by
the formation of a liquid skin on the melting nanoparticles. The liquid
skin thickness increases during melting gradually consuming the solid
particle core. Transmission electron microscopy studies, such as the one
discussed in "Shape Transformation and Surface Melting of Cubic and
Tetrahedral Platinum Nanocrystals" by Z. L. Wang, J. M. Petroski, T. C.
Green and M. A. El-Sayed, J. Phys. Chem. 102, (32) 6145-6151 (1998), have
established that 8 nanometer platinum nanoparticles begin to melt at
about 600° to about 650° C., which is a much lower
temperature than the melting point of bulk platinum at 1769° C. At
about 500° C., cubic particles change their shape to a spherical
shape with surface melting occurring at about 600° C. to about
650° C. The molten layer surrounding solid cores of platinum
nanocrystals is about 1 nm in thickness at 600° C. and the
thickness increases with temperature as the nanoparticles continue to
melt. The "melting point depression" abbreviated as MPD is a
thermodynamically driven phenomenon and can be explained by a drastic
increase in the surface area/volume ratio in nano-particulate materials
and the corresponding increase in their specific surface energy. This
leads to a size-related dependence of melting temperature that is roughly
close to 1/d functionality, where d is the mean particle size, and
contains surface tension coefficients, latent heat of melting and the
molten skin thickness as parameters.

[0009]Table 1 presents some experimental data illustrating the difference
in melting temperatures for nanoparticles and the corresponding bulk
metals and semiconductors.

[0010]Onset of surface melting occurs usually at temperatures even lower
than the temperature at which the entire nanoparticle melts. It can be
speculated that the "molten shells" of the pre-melted nanoparticles work
as "a lubricant" inducing higher mobility of the particles and higher
diffusion rates and hence facilitating densification at temperatures much
lower than 0.6 of the melting point (Tm).

[0011]It can be further speculated that thermodynamic considerations
explaining the mechanism of MPD described above should hold for ceramic
nanoparticles as well. Nevertheless, the MPD effect is not very well
studied in ceramics for obvious reasons--even the depressed melting point
anticipated for ceramic nanoparticles will still be very high making it
extremely difficult to conduct observations similar to those for metals
and semiconductors described above in Table 1.

[0012]For example, the melting point (TM) for pure alumina and
zirconia are 2050° C. and 2700° C., respectively, and
therefore the MPD effect of the order of 0.5TM will result in
melting temperatures for nano-alumina and nano-zirconia particles of
about 1025° C. and 1350° C. However, there are some
indirect indications that MPD does occur in nanoceramics such as
extremely low sintering temperatures for nanopowders as reported in R. A.
Kimel, Aqueous Synthesis and Processing of Nanosized Yttria Tetragonally
Stabilized Zirconia, Ph.D. Thesis, The Pennsylvania State University, the
Graduate School, the College of Earth and Mineral Sciences, (2002) and in
G. Skandan, H. Hahn, M. Roddy and W. R. Cannon, "Ultrafine-Grained Dense
Monoclinic and Tetragonal Zirconia," J. Am. Ceram. Soc., vol. 77, no. 7,
pp. 1706-10 (1994), which are both hereby incorporated by reference.

[0013]These studies reported onset of densification at surprisingly low
temperatures of about 0.3TM as well as a surprising and unique
ability of nanoceramics to be translucent at fairly high levels of
porosity.

[0014]Some studies reported extreme difficulty in sintering nanoceramics
to full density due to rapid grain growth. For example, Skandan et al.
(cited above) observed that grains grew 15 times the initial particle
size in the case of nano-zirconia. The other major obstacle encountered
with the use of nanoparticles in the fabrication of dental articles is
related to difficulties in the consolidating of bulk shapes using
conventional methods like powder compaction and slip-casting. It the
scope of the present invention to utilize the advantages of
nanoparticulate ceramics while successfully overcoming the obstacles
currently hampering use of such nanoparticulates as dental ceramics.

[0015]It is desirable to provide dental ceramics having low sintering
temperatures and high strengths. It would be beneficial to provide dental
ceramics having sintering temperatures that are low enough to be sintered
in existing dental furnaces, yet maintaining high strength and
translucency. It is most desirable to provide processing techniques for
dental ceramics that result in fully densified dental ceramics.

SUMMARY OF THE INVENTION

[0016]These and other objects and advantages are accomplished by the
ceramic powders of the present invention which are manufactured into
dental articles. The ceramic powders may include sintering agents,
binders and other similar additives to aid in the processing of the
ceramic powder into a dental article. The ceramic powders are comprised
of nanoparticulate crystallites. The ceramic powders may be processed
into the dental article using various methods including, but not limited
to, injection molding, gel-casting, slip casting, or electroforming, hand
forming, cad/camming and other various rapid prototyping methods. The
ceramic powder may be formed into a suspension, pellet, feedstock
material or a pre-sintered blank prior to forming into the dental
article.

[0017]Dental articles are produced using relatively low sintering
temperatures to achieve high density dental articles exhibiting strengths
equal to and greater than about 700 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]Features of the present invention are disclosed in the accompanying
drawings, wherein similar reference characters denote similar elements
throughout the several views, and wherein:

[0019]FIG. 1 is a schematic diagram showing the structure of particles
described herein;

[0022]FIG. 4 is a schematic diagram of sample geometry and an orthogonal
coordinate system used for sintering shrinkage measurements;

[0023]FIG. 5 is a graph showing shrinkage of outside diameter Lx
versus shrinkage of the height of cups made out of conventional zirconia
powder and nanosized zirconia powder (NP-2); and

[0024]FIG. 6 is a graph showing shrinkage of outside diameter Ly
versus shrinkage of the height of cups made out of conventional zirconia
powder and nanosized zirconia powder (NP-2).

DETAILED DESCRIPTION OF THE INVENTION

[0025]This invention provides particulate materials that can be processed
into dental restorations using both the most sophisticated state-of-the
art technologies such as solid free form manufacturing (SFF) methods as
set forth in U.S. Pat. No. 6,322,728, and copending commonly owned U.S.
patent application Ser. Nos. 09/972,351 (US 2002/00335458), now U.S. Pat.
No. 6,994,549, Ser. No. 10/053,430 (US 2002/0125592), now U.S. Pat. No.
6,808,659, and Ser. No. 09/946,413 (US 2002/0064745), now U.S. Pat. No.
6,821,462, all of which are hereby incorporated by reference, as well as
manual techniques similar to classic methods of building porcelain jacket
crowns on a refractory die (e.g. OPC® Low Wear porcelain jacket crowns
made from powder or jacket crowns made using tape-cast ceramic sheets as
described in U.S. Pat. No. 5,975,905, which is hereby incorporated by
reference) or slip casting a ceramic slip onto a porous die/mold.

[0026]The ceramic particulate materials of the present invention have
complex hierarchical architecture with three levels of structural
organization: nano-, micro-, and macro-level as shown in FIG. 1. On the
nano-level (≦20 nm), the structure is based on nano-crystallites
depicted at 10 as elemental building blocks. On the micro-level (0.1-20
microns), the structure is formed from polycrystalline particles or
agglomerates comprised of clusters of nanocrystallites depicted at 12. On
a macro-level (20-200 microns), polycrystalline particles are
agglomerated into granules depicted at 14. These granules 14 are made by
spray-drying or fluidized bed agglomerating polycrystalline particles
comprised of nanoparticles of one or more kinds of materials including,
but not limited to, metallic and ceramic materials which may be fully or
partially calcined or still in the form of organic/inorganic precursors.
Sintering aids are depicted interstially at 16. The advantage of using
nanoparticles is their drastically different sintering behavior
associated with MPD. In much of the scientific literature, such as in,
"Melting of Isolated Tin Nanoparticles" by T. Bachels, H-J Guntherodt,
and R. Schafer, Phy. Rev. Lett., 85, (6), 1250-1253, (2000), which is
hereby incorporated by reference, this effect is also referred to as
"size dependence of melting temperature" in nano-materials. As a result
of this mechanism, sintering of nanoparticles is speculated to be aided
by the occurrence of surface pre-melting and hence, controlled by
capillary forces. Beneficial utilization of capillary forces through the
hierarchical architecture of ceramic powders comprising nanoparticles is
an essential feature of this invention. The hierarchical architecture of
nanocrystalline powders of this invention is specifically engineered to
1) aid consolidation of particulates into green shape; 2) take advantage
of capillary effects during sintering (i.e. liquid phase sintering and
surface melting of nanoparticles) to maximize particle rearrangement,
enhance sintering kinetics and lower the sintering temperature; and 3)
control the size, distribution and morphology of the residual porosity.

[0027]Examples of metallic powders useful herein include, but are not
limited to Si, Al, Mg, Zr, Y, Ce, Ta and mixtures thereof. These metals
are primarily chosen because they oxidize easily and form glass-forming
oxides SiO2, Al2O3, MgO, ZrO2, Y2O3,
CeO2 and Ta2O5 that facilitate liquid-phase formation
during sintering. Most of these oxides are currently used as sintering
aids or dopants in the manufacture of high-performance ceramics such as
alumina, zirconia, silicon nitride and SIALON ceramics. The advantage of
adding these elements in metallic rather than oxide form is that as
nanophase metal particles they are extremely reactive, have high enthalpy
of oxidation, i.e., generate highly localized heat upon oxidation, and
provide good coupling for microwave energy.

[0028]Examples of ceramic nanocrystalline powders useful herein include,
but are not limited to, oxide ceramics such as various forms and
modifications of zirconia, alumina, titania, silica, magnesia, yttria,
ceria and solid solutions or mixtures thereof.

[0029]The metallic powders and ceramic nanocrystalline powders of the
present invention have sintering temperatures lower than about
1300° C. and preferably lower than about 1200° C. and most
preferably not exceeding about 1100° C. Sintering temperatures of
lower than 1300° C. are the most economical since sintering can be
carried out in the most common resistance-heated furnaces having metallic
heating elements. Each dental lab has at least one burn-out furnace with
a maximum continuous operating temperature of at least 1100° C.
and a porcelain furnace with a maximum operating temperature of about
1200° C. The essential feature of this invention is that these
powders can be processed into dental restorations using both the most
sophisticated state-of-the-art technologies such as Solid Free Form
Manufacturing (SFF) methods, also known as Rapid Prototyping (RP), as
well as manual techniques similar to classic methods of building
porcelain jacket crowns on a refractory die, or jacket crowns made using
tape-cast sheets or slip casting a ceramic slip on a porous mold.
Examples of SFF/RP methods include stereolithography (SLA) and
photo-stereo-lithography including Digital Light Processing (DLP) and
Rapid Micro Product Development (RMPD) mask technique, selective laser
sintering (SLS), ballistic particle manufacturing (BPM), fusion
deposition modeling (FDM), multi-jet modeling (MJM) and three-dimensional
printing (3DP).

[0030]Particle size distribution of these powders made of granules and
polycrystalline particles is designed to improve handling for making
restorations by hand or to optimize powder bed or feed-stock
characteristics for specific SFF methods used. Engineered particulate
materials of the present invention have complex hierarchical architecture
with three levels of structural organization: nano-, micro-, and
macro-level as shown in FIG. 1. On the macro-level the particulates are
formed into a nearly spherical shape with a diameter from about 10
microns to about 500 microns, more preferably from about 20 to about 200
microns. These spherical particles should preferably be solid, not
hollow. The size distribution of powders useful herein should be
optimized for the specific forming technique. For example, for building
by hand, the particle size distribution should be bimodal with the
fraction of finer particles fitting in the interstitials between the
fraction of coarser particles. The ratio of the mean diameter of coarser
particles (Dc) to the mean diameter of finer particles (Df),
Dc/Df, should be more than 3 and preferably more than 6,
whereas the relative amount of finer particles is from about 10 wt % to
about 25 wt %. An example of particle size distribution is shown in FIG.
2. Curve 20 depicts particle size distribution of nanophase powder
facilitating manual build-up and curve 22 depicts typical volume based
distribution for dental porcelain with good "handling."

[0031]In addition to using nanocrystalline powders to induce low sintering
temperatures, sintering aids and binders/additives may be mixed with the
nanocrystalline powders to further facilitate the action of capillary
forces to aid in powder consolidation and sintering. Binders used herein
may include any known binder used in conventional powder processing
methods and may be compounds or mixtures of compounds activated by heat,
light, or other types of radiation or by chemical reaction. Examples of
binders/additives include, but are not limited to, polymeric binders,
plasticizers, surfactants and dispersants such as polyacrylic binders,
polyvinyl alcohol (PVA) binders, polyvinyl butyral binders, stearic and
oleic acids, silanes, and various natural and synthetic waxes such as
paraffin wax, polyethylene wax, carnauba wax, and bee's wax.

[0032]In accordance with a method herein, ceramic powder comprising
nanocrystals is mixed with a metallic sintering aid comprising metallic
micro- and/or macro-size particles. Other sintering aids and binders may
also be added to the mixture. During mixing, the nanocrystalline powders
become coated with the additives using any of the available
coating/agglomeration techniques including, but not limited to, spray
drying, fluidized bed agglomeration methods, dry and wet milling and
mechanical alloying. In one of the preferred embodiments of the present
invention, the additives comprise metallic particulates. In another
embodiment, the additives comprise ceramic nanophases and/or
nanocrystallites such as grain growth inhibitors. These sintering
aids/additives facilitate the thermal sintering/densification process.

[0033]After mixing, the mixture is then sintered at a temperature of less
than about 1300° C., preferably at a temperature of less than
about 1200° C. and most preferably in the range of temperatures
from about 800° C. to about 1150° C.

[0034]In yet another embodiment, microwave processing is used to densify
the particulate by sintering through the absorption of microwave energy.

[0035]Processes occurring during the melting of a material comprising
nanoparticles as described above are somewhat similar to the processes
occurring during liquid phase sintering as described in Fundamentals of
Ceramics, M. Barsoum, McGraw Hill (1997). In both cases, the defining
factor is the presence of liquid and therefore the entire process is
controlled by capillary forces. In contrast however, during liquid phase
sintering the liquid phase is formed due to the addition of sintering
aids, and during the sintering of nanoparticles, formation of the liquid
skin on nanoparticles occurs by the mechanism of surface melting,
intrinsic to nanophase materials. This invention takes advantage of both
intrinsic liquid formation (due to surface melting of nanoparticles) and
extrinisic liquid formation (due to sintering aids). The nanocrystalline
structure of the polycrystalline particles are combined with sintering
aids when they are agglomerated into granules. The granules themselves
are coated with a coating comprising sintering aids and agents to aid
capillary forces during sintering as well as in the forming of the shape.
For example, handling of the powder/liquid paste-like mixture for manual
wet-condensing of ceramic powder on a refractory die is primarily
controlled by capillary forces.

[0036]Particulate material comprised of nanoparticles may behave in ways
similar to glass-ceramic material due to a very significant fraction of
relatively disordered material on grain boundaries. In addition,
extremely high specific surface energy associated with nanoparticles
greatly increases the driving force for densification. A high fraction of
grain boundaries substantially alters sintering behavior of nano-sized
ceramics compared to that of conventional micron-sized ceramics. Surface
melting of nanoparticles, resulting in liquid skin formation around
nanoparticles, induces and promotes the mechanisms of sintering
previously associated with liquid phase sintering such as particle
rearrangement and solution-precipitation. At the same time,
grain-boundary diffusion sintering mechanisms are greatly enhanced due to
the enormous surface area of the nanoparticles. Normally, the presence of
agglomerates inhibits densification during solid-state sintering,
however, with added mechanisms of liquid phase sintering and surface
melting of nanoparticles, deliberate granulation of powder is an
essential feature of this invention that facilitates beneficial capillary
effects during green shape fabrication and sintering.

[0037]It is expected that hand-built restorations will have some residual
porosity after final sintering, however, the architecture of the powder
is designed to minimize this residual porosity and spatially coordinate
it to minimize its adverse effect on mechanical properties. It is now
recognized that porosity is practically an unavoidable element of
microstructure and porous ceramics are not necessarily weak, as stated in
"Fracture Energy of an Aligned Porous Silicon Nitride," by Y. Inagaki, T.
Ohji, S. Kanzaki and Y. Shigekaki, J. Am. Ceram. Soc., 83 (7), 1807-1809,
(2000). Porosity as an engineered element of the microstructure of the
materials of the present invention can be controlled and spatially
organized through engineered hierarchy of the starting particulate
material. It is well known in the art that the critical flaw size causing
brittle fracture of ceramics often scales with the particle size of the
starting powder. In materials of the present invention, the powder is
preferably spherical in shape promoting better flowability of the powder.
In the powder herein, the pore size scales with the diameter of
interstitial sites formed between the particles of the powder providing
that the powder was carefully condensed or compacted and attained the
maximum green density of the compact. The pore size and spatial
distribution will be defined by the size and spatial distribution of
interstitials between the particles. For example, for a spherical powder
with particle size distribution shown in curve 1 of FIG. 2 the largest
pore diameter will be defined by the size of the largest interstitial
between the smallest spherical particles, which in this case is about 20
μm. The largest, octahedral interstitial in close packed arrangement
of 20 μm spheres will be ( 3-1)μm×20=0.732×20
μm=14.64 μm.

[0038]The equation σ=KIC/(Ya1/2) calculates the strength
based on the value of fracture toughness and the critical flaw size,
where [0039]KIC is the fracture toughness; [0040]Y is the
geometric factor; [0041]σ is the fracture strength; [0042] is the
square root; and [0043]2a is the equivalent crack length associated with
the critical flaw.

[0044]For a yttria-stabilized tetragonal zirconia polycrystals (YTZP)
material with KIC of 6-9 MPam1/2, and a geometric factor (Y) of
2, strength will most likely be well in excess of 700 MPa.

[0045]Nanocrystalline particulate ceramic materials of this invention are
supplied as free-flowing powder, pre-sintered blanks, feed-stock (for
injection molding) and suspensions for slip-casting or electroforming and
using fabrication techniques described below to provide materials with
flexure strength of at least 700 MPa and exceeding 1 GPa which is more
than enough even for multi-unit posterior restorations and cantilever
bridges.

[0046]The following examples illustrate the invention.

Example 1

[0047]Commercially available TZ-3Y-E (which is a yttria-stabilized
tetragonal zirconia powder) powder manufactured by TOSOH Corporation
(Japan) with a crystallite size of about 30 nm can be sintered to full
density at temperatures as low as 1350° C., which is 150°
C. lower than the sintering temperature for conventional
yttria-stabilized tetragonal zirconia polycrystals (YTZP) powder. Onset
of sintering normally occurs at about 0.6 of the melting point (Tm) and
the MPD effect described above results in a corresponding decrease in
sintering temperature for nanopowder compared to conventional micron-size
ceramic. If the size of the crystallites is reduced three times to about
10 nm, the anticipated reduction in the sintering temperature will be
about 450° C., i.e., YTZP powder comprised of 10 nm nanoparticles
is sinterable at about 1050° C. Thus, this powder may be sintered
in a regular burn-out furnace with the maximum operating temperature of
1100° C. Example 2 below further illustrates the viability of
sintering nanosized zirconia powders to nearly full density at
temperatures lower than 1300° C. and preferably lower than
1100° C.

Example 2

Low Temperature Sinterability of Nano-Zirconia Powders

[0048]Two commercially available nano-sized 3 mol % yttria stabilized
zirconia powders were obtained from NanoProducts Corporation (Longmont,
Colo. 80504, USA). The physical characteristics of these powders are
listed in Table 3. Also listed are the properties of TZ-3YS-E powder
available from Tosoh Corporation (Tokyo, Japan), a conventional so-called
"easy sintering" 3 mol % yttria-stabilized tetragonal zirconia powder
that was used for baseline comparison. The nano-powders were mixed with
5-10 wt. % PVA binder (Elvanol 50-42, Dupont) with a mortar and pestle,
and then sieved through an 80 mesh screen resulting in free-flowing,
pressable powder. The conventional zirconia powder was pressable to begin
with, as it contained binder.

[0049]The powders were pressed into pellets using a double action die and
then vacuum bagged and cold isostatically pressed (CIPed) at 400 MPa to
remove any green density gradients. The resulting pellets were
approximately 3 grams in weight and measured about 12 mm in diameter and
about 7 mm in height. The green density of conventional and nanosized
zirconia pellets were approximately the same, 54±0.2% and
52.6±1.2%, respectively. The pellets were then burned out by heating
at a rate of 2° C./m to 700° C. and holding for 2 hours.
The pellets were subsequently sintered at 1200-1300° C. for 2
hours with a heating and cooling rate of 4° C./m. Density of the
sintered pellets was measured by the Archimedes method using water as the
immersion medium. The percent theoretical density was calculated using a
theoretical density value of 6.05 g/cm3. These results are shown in
FIG. 3 and demonstrate that at temperatures below 1250° C., the
nano-sized zirconia samples exhibit enhanced sintering behavior at lower
temperatures versus the conventional zirconia sample. For example at
1225° C. the NP-2 pellets densified to 94.1±0.2% which compares
to 86.5±0.2% for the conventional zirconia sample. This improved
sintering behavior is attributed to the smaller crystallite and particle
size of the NP-1 powder (see Table 3). This sintering enhancement due to
smaller crystallites/particles is also reflected by the data at
1200° C., which shows a progressive increase in density from the
conventional zirconia sample (74.5±0.3%) to the nanparticulate
zirconia samples, NP-1 (83.1±0.1%) and NP-2 (85.8±0.1%). To reduce
sintering temperatures below 1000° C. the average particle size
should be reduced to below about 8 microns as demonstrated by Kimel and
Skandan et al. (cited above).

[0050]Example 3 further illustrates that some of these nanopowders can be
sintered isotropically using simplified cup shape geometry.

Example 3

[0051]Using the methods described in example 1, green cylindrical shaped
bodies with dimensions of ˜12 mm diameter by ˜12 mm height
were formed out of the NP-2 and conventional zirconia powders. A 5 mm
diameter×8 mm deep hole was machined into the green bodies yielding
a "cup" geometry. This geometry was chosen since it simulated a dental
coping. After recording the orthogonal dimensions, L.sub.(x,y,z) where x
and y refer to the diameters taken at a 90° rotation to each other
and z refers to the height as illustrated in FIG. 4, the green bodies
were burned out by heating at a rate of 2° C./m to 700° C.
and holding for 2 hours, and subsequently sintering at 1225° C.
for 2 h with a heating and cooling rate of 4° C./m. The dimensions
of the sintered bodies were recorded and the sintering shrinkage was
calculated. These results are shown in FIGS. 5 and 6. These data show
that greater shrinkages were achieved for the nanosized zirconia powder
versus the conventional zirconia, which agrees with the sintering results
shown in Example 2. Additionally, as manifested by the data points
falling on the line of isotropic shrinkage represented by the dashed
lines, these results demonstrate that like the conventional zirconia the
nanosized zirconia also densifies and shrinks isotropically during
sintering.

[0053]The blend of 0.5 wt % of NP-3 and 99.5 wt % of NP-2 nano-powders was
mixed with 5-10 wt % PVA binder (Elvanol 50-42, Dupont) with a mortar and
pestle, and then sieved through an 80 mesh screen resulting in
free-flowing, pressable powder. The powder was vacuum bagged and cold
isostatically pressed at 400 MPa to produce green billets of about 1-2
inches in diameter and about 5-10 inches in length. Following the cold
isostatically pressing step, the outer layer of the billets were removed
by turning to eliminate any green density gradients that may have existed
in the outer layer. The billet was further sectioned into shorter
cylinders of about 30 mm in diameter and about 50-60 mm in height. The
cylinders were then debinderized and pre-sintered to about 50%
theoretical density in a two step firing cycle comprising heating at the
rate of 1° C./minute to about 700° C. and holding for about
2 hours at this temperature followed by a 2 hour hold at about
900° C. The attained bisque densities and the anticipated
Bisque-to-Final linear shrinkages were calculated for each individual
block based on diameter and height measurements before and after
pre-sintering.

[0054]The pre-sintered cylinders are subsequently used to mill the
enlarged frameworks for dental restorations. Each framework is enlarged
based on the linear shrinkage factor calculated for each individual
pre-sintered cylinder from which the framework is milled. The milled
frameworks are sintered at 1250° C. for 4 hours with a heating
rate of 2° C./minute to densities exceeding about 95% theoretical
density as determined by the Archimedes method. The isotropic shrinkage
in the frameworks is confirmed by fitting frameworks on the original
master model. Some of the frameworks were layered by 3G Porcelain
(Pentron® Laboratory Technologies, LLC, Wallingford, Conn.) to
demonstrate the finishing steps typical in fabrication of aesthetic
all-ceramic dental restorations.

where L is the linear shrinkage. It was observed that sintering of the
compacts of the nanopowders capable of isotropic sintering results in
nearly fully densified articles with the average grain size noticeably
larger than 100 nm. Two major difficulties in processing dental articles
using nanopowders were revealed: (1) the consolidation of bulk shapes by
conventional methods such as powder compaction and slip-casting; and (2)
sintering to achieve densities in excess of 95%.

[0056]To overcome the first processing obstacle mentioned above, solid
free form manufacturing methods such as rapid prototyping or solid
imaging are utilized indirectly in combination with other processing
techniques such as injection molding/heat-pressing, various coating or
deposition techniques such as gel casting, slip casting, slurry casting,
pressure infiltration, dipping, colloidal spray deposition, direct
coagulation as described in U.S. Pat. Nos. 5,667,548, 5,788,891 and
5,948,335, which are hereby incorporated by reference, and electroforming
or electrophoretic deposition techniques. While SFF methods are used to
fabricate enlarged substrates, dies and molds, any of the above listed
techniques can be used to form nanoparticulate materials of these
invention into green shapes conforming to these substrates or molds. The
electroforming is a preferred method since it utilizes suspensions which
are particularly beneficial for the nanomaterials herein described. Many
of the nanoparticulate materials described herein are more readily
obtained as well-dispersed suspensions rather than free-flowing powders.
Example 5 illustrates electroforming as the preferred method of
depositing ceramic nanoparticulates onto enlarged dies produced by one of
solid free form manufacturing methods. Yet another preferred technique is
low-pressure injection molding into negative molds of the rapid
prototyped models, or alternatively existing heat pressing equipment can
be used for pressing into refractory investment molds produced by lost
wax technique. Example 6 illustrates low-pressure injection molding as
another preferred method of forming ceramic nanoparticulates using
enlarged molds produced by one of SFF methods.

[0057]To alleviate the second processing obstacle mentioned above, the
nanopowders herein are agglomerated with sintering aids such as Si, Al,
Mg, Zr, Y, Ce, Ta and mixtures thereof, and grain growth inhibitors such
as Cr, Ti, Ni, Mn and mixtures thereof. Depending on the subsequent
processing steps, these additions can be added in their elemental
(metallic) form or in the form of oxides, salts, organometallic
compounds, or other precursor compounds, in the form of colloids, powders
and specifically nanopowders. To further lower the melting temperature,
the inclusion of the above-mentioned additives as nanopowders or
precursor compounds, is most preferred.

Example 5

[0058]An optical scanner, ZFN D-21, available from ZFN (Zahntechnisches
Fraszentrum Nord GmbH & Co. KG, Warin, Germany) is utilized to scan
master models (dies) made from impressions comprising preparations for
bridges and crowns. Three-dimensional CAD software provided with a ZFN
D-21 scanner is used to design frameworks and copings corresponding to
these master models (dies). 3D CAD files (solid models) of these
frameworks and copings are enlarged using the linear shrinkage
coefficient corresponding to the anticipated sintering shrinkage of the
nanozirconia materials of the present invention, saved as
stereolithography (.STL) files and transferred to a computer interfaced
with an RP (Rapid Prototyping) machine such as Perfactory® Mini
available from Envision Technologies GmbH (Marl, Germany). This machine
utilizes a photostereolithography process also known as digital light
processing (DLP) to build three-dimensional objects from a light curable
resin. Fifteen units are built at the same time layer by layer with an
individual layer thickness of about 50 microns. Individual units are
separated, attached to copper wire electrodes and coated with conductive
silver paint (silver lacquer) available from Gramm GmbH or Wieland
Dental+ Technik GmbH & Co., KG (Pforzheim/Germany). AN electroforming
unit, such as AGC® Micro Plus (Wieland Dental+ Technik GmbH & Co.
KG), is used to deposit a dense layer of yttria-stabilized zirconia
polycrystals (YTZP) from an ethanol based suspension as described in
Example 3 of U.S. Pat. No. 6,059,949, which is hereby incorporated by
reference. An electroforming suspension is prepared by suspending NP-2
zirconia powder available from NanoProducts Corp. (Longmont, Colo.) in
pure ethanol with addition of 0.05% vol. acetyl acetone dispersant and
0.1% vol. of 5% wt. PVB (polyvinyl butyral binder) in pure ethanol.
Alternatively, an ethanol-based suspension is prepared from an aqueous
suspension comprising tetragonal nano-zirconia particles of about 8 nm
average particle size. First, aqueous suspensions of YTZP having a
crystal size of about 8 nm are prepared via precipitation from
homogeneous solutions using complexation chemistry techniques. Zirconium
and yttrium salts, ZrO(NO3)2.xH2O (zirconyl nitrate,
Aldrich Chem., Milwaukee, Wis.) and Y(NO3)2.6H2O (yttrium
nitrate hexahydrate), Aldrich Chem., Milwaukee, Wis.) are each dissolved
in CO2-free deionized water in the appropriate amounts to achieve
0.5 M solutions of each. These are then mixed, in the appropriate ratio
to yield the desired mol. % of Y2O3 in the final powder, with
the complexing agent bicine (www.sigmaaldrich.com) (2:1 bicine:Zr (mol)).
The pH of this feed solution is adjusted to about 13 by additions of
TEAOH (Tetraethylammonium Hydroxide, Aldrich Chem., Milwaukee, Wis.), and
the solution is then put in a teflon-lined hydrothermal vessel (Parr
Instrument Company, Moline, Ill.), which is heated to 200° C. for
8 hours to hydrothermally synthesize YTZP crystals of about 8 nm in size.

[0059]Aqueous suspensions are converted into alcohol-based suspensions by
centrifuging and then redispersing in ethanol. The average thickness of
the electrophoretic coating is about 0.5-0.6 mm. Following electroforming
of the powders onto the substrates, sintering is carried out in a Deltech
furnace using a two-step firing cycle comprising heating rate of
1° C./min to about 450° C., holding at this temperature for
2 hours to remove organics, further heating at a rate of 1°
C./min. to 900° C.-1100° C. and holding at this temperature
for about 2 hours. Densities in excess of 90% of theoretical density can
be achieved.

Example 6

Low-Pressure Injection Molding (LPIM) with Peltsman Unit

[0060]An optical scanner, ZFN D-21, available from ZFN (Zahntechnisches
Fraszentrum Nord GmbH & Co. KG (Warin, Germany) is utilized to scan
master models (dies) made from impressions comprising preparations for
bridges and crowns. 3D CAD software provided with a ZFN D-21 scanner is
used to design frameworks and copings corresponding to these master
models (dies). 3D CAD files (solid models) of these frameworks and
copings are enlarged using the linear shrinkage coefficient corresponding
to the anticipated sintering shrinkage of the nanozirconia materials of
the present invention, saved as stereolithography (.STL) files and
transferred to a computer interfaced with an RP (Rapid Prototyping)
machine such as Perfactory® Mini available from Envision Technologies
GmbH (Marl, Germany). This machine utilizes photostereolithography
process also known as digital light processing (DLP) to build 3D objects
from a light curable resin. Fifteen units are built at the same time
layer by layer with an individual layer thickness of about 50 microns.
Individual units are separated and molded in a liquid silicone rubber
(Silastic® M RTV Silicone Rubber from Dow Corning Corporation) which
is castable and easily demolded after curing to produce negative molds
for low-pressure injection molding. It should be noted that instead of
using silicone negative molds, the molds for LPIM can be designed and
fabricated directly using the Perfactory® Mini RP machine and the
supplied software.

[0061]Feedstock containing nanosized zirconia for injection molding is
prepared from a binder comprised of 75 wt % of paraffin wax (melting
point of 49°-52° C.), 10 wt % of polyethylene wax (melting
point of 80°-90° C.), 10% of carnauba wax (melting point of
80°-87° C.), 2 wt % of stearic acid (melting point of
75° C.) and 3 wt % of oleic acid (melting point of 16° C.)
readily available from a number of suppliers. Nanosized zirconia having a
crystallite size of about 19 nm and particle size of about 15 nm
(available as Product Number ZR3N3269 from NanoProducts Corp., Longmont,
Colo. 80504, USA) is used. The mixing is done directly in a low pressure
molding (LPM) machine, (Model MIGL-33 available from Peltsman
Corporation, Minneapolis, Minn.) at a temperature of 90° C. The
feedstock mixture is comprised of about fifty percent (50%) by volume of
a binder. Once the feed stock mixture is thoroughly mixed it is injected
into the cavity of the silicone rubber molds at a pressure of
approximately 0.4 MPa and a temperature of approximately 90° C.
The injection-molded green part is then demolded from the silicone mold,
which is done easily due to elasticity of the silicone. Green densities
of approximately 50% were achieved. The green bodies were debinderized
and sintered to nearly full density as described above.

Example 7

Injection Molding with Autopress

[0062]Feedstock containing nanosized zirconia for injection molding is
prepared from a binder comprised of paraffin wax, with minor proportions
of polyethylene wax, carnauba wax, stearic and oleic acids using the same
formulation as used in Example 6. Nanosized zirconia having a crystallite
size of about 19 nm and a particle size of about 15 nm (available as
Product Number ZR3N3269 from NanoProducts Corp., Longmont, Colo. 80504,
USA) is used. The mixing is done in a KitchenAid Professional 5 mixer
(St. Joseph, Mich.) in a bowl continuously heated to 90° C., which
is above the melting point of the binder. Heating is achieved using a
high temperature heat tape available from McMaster-Carr (New Brunswick,
N.J.). The heat tape is wrapped around the mixer bowl to provide heat to
the bowl. After cooling to room temperature, the resulting mix is crushed
into powder to a 60 mesh (250 μm) particle size using a mortar and
pestle. This powder is then ready for injection into the cavity of a
mold. Additionally, the mix can be cast into pellets by pouring into a
metal "clam-shell" mold, while still in the molten state.

[0063]Previously acquired stereolithography (*.STL) files of bridge
frameworks and crown copings were sent to microTEC, Bismarckstrasse 142 b
47057 Duisburg, Germany) for production of the enlarged replicas using
RMPD®-mask technology via toll rapid prototyping service available
through microTEC's website. The replicas were fabricated in a layer
thickness of twenty five microns (25 μm) from photo-curable resin.

[0064]The resulting replicas are invested in Universal® Refractory
Investment (available from Pentron® Laboratory Technologies, LLC,
Wallingford, Conn.). After the investment has hardened, the resin
replicas inside are eliminated by placing it into a preheated furnace
thereby burning off the resin, resulting in a mold cavity for forming the
dental article. The injection molding feedstock, in free-flowing granule
or pellet form, as described above, is then placed into the investment
mold assembly, which is then transferred into the pressing unit. It is
pressed into the investment ring using an AutoPressPlus®
(Pentron® Laboratory Technologies, LLC, Wallingford, Conn.). having
an external alumina plunger Pressing is done at approximately 90°
C., and after cooling the pressed green part is then carefully divested
by sand-blasting with glass beads at a pressure of 15 psi and the plunger
and mold are disposed of. Green densities of approximately 50% were
achieved. The green bodies were debinderized and sintered to nearly full
density as described above.

[0065]It should be noted that in all the cases described in Examples 1-7
it was observed that while the ceramic portion of the starting powder,
suspension or feedstock consists of crystallites with average sizes of
less than 20 nm, the sintered dental articles have average grain sizes
within the range from about 100 nm to about 450 nm. It is the nature of
the materials of the present invention to exhibit substantial coarsening
concurrent with densification wherein the final grain size is about 10-20
times larger than the starting crystallite size.

[0066]Though not within the scope of the present invention which is
directed towards sintering ceramic dental articles comprising nanopowders
to nearly full density, nevertheless, it should be noted that the
injection molding technology described in Examples 6 and 7 can be used to
produce dental articles even if access to RP machines is not available.
In the latter case, if is not possible to make enlarged replicas and
green bodies fabricated therefrom as described in Examples 6 and 7, the
articles will have to be presintered without shrinkage and glass
infiltrated as described in U.S. Pat. Nos. 4,772,436 and 5,910,273, which
are hereby incorporated by reference. In the case of YTZP zirconia cores,
3G porcelain (Pentron® Laboratory Technologies, LLC, Wallingford,
Conn.) can be used for both glass infiltration and esthetic layering of
the resulting glass-infiltrated cores.

[0067]While various descriptions of the present invention are described
above, it should be understood that the various features can be used
singly or in any combination thereof. Therefore, this invention is not to
be limited to only the specifically preferred embodiments depicted
herein.

[0068]Further, it should be understood that variations and modifications
within the spirit and scope of the invention may occur to those skilled
in the art to which the invention pertains. Accordingly, all expedient
modifications readily attainable by one versed in the art from the
disclosure set forth herein that are within the scope and spirit of the
present invention are to be included as further embodiments of the
present invention. The scope of the present invention is accordingly
defined as set forth in the appended claims.